To ensure that one machine can be used economically for different sizes of containers, bottles, tins or similar items, so-called format adapters are used, which are adapted individually to a wide variety of container shapes and act as adapters between the container and the machine. Depending on the shape of the container, a correspondingly designed format adapter is used to transport the container inside the machine, fix it in position or to align it for printing. Labelling containers such as bottles or other packagings using inkjet printers is known in the art. In such cases, the container to be printed is placed on a turntable in a printing station and centred. The container is then rotated by the turntable. A region of the surface of the container to be printed is then passed by one or more printing heads arranged in the station, which heads print the container by spraying a printing medium from nozzles onto the surface of the container while said container rotates in relation to the printing head. DE 10 2009 058 222 A1, for example, describes a known machine for printing containers in which a container to be printed is placed on a turntable which is rotated about an axis of rotation by a servomotor. A clamping device fixes the container in position before printing commences.
In printing methods such as the drop-on-demand method, the surface of the container to be printed may be printed at a specific maximum droplet sequence frequency depending on the capacity of the respective printing head. In the case of a print image resolution of 360 dpi, the current maximum achievable frequency in the prior art is typically 6000 dots per second. As a result of this limit, the relative speed at which the surface to be printed passes by the printing head is limited. Given the above-mentioned ejection frequency, this results in a maximum relative speed between the printing head and the printing region on the surface of approximately 423.333 mm/s or 60.666 inch/s. A higher relative speed would reduce the quality of the print image.
However, in sectors such as the beverage industry, processing volumes of 36,000 bottles per hour for example are assumed. The throughput when printing the bottles must therefore be correspondingly high, and a high print quality must still be guaranteed at the same time. In order to meet the requirement for a rapid throughput, it is advantageous if the capacity of the printing stations can be fully utilised. In particular, this can be achieved by maintaining the relative speed between the printing surface and the printing head in consideration of the printing head capacity in the region of the maximum relative speed in order to maintain the overall throughput at a high level.
However, consideration must also be given to the fact that, in the case of a body, such as a bottle, rotating about an axis of rotation, the relative speed between the region on the surface of the container to be printed and the printing head is dependent on the distance between the location to be printed on the surface of the three-dimensional container and its axis of rotation.
A wide bottle having a correspondingly large radial extent has a larger peripheral speed than a narrow bottle having a relatively small radial extent at the same rotational speed or angular velocity. The relative rotational speeds at the printing head vary at the same speed or angular velocity.
In order to process containers of different diameters but at the same capacity utilisation rate for the printing head, the appropriate rotational speed or angular velocity must be set for every possible container diameter in order to achieve a peripheral speed in the printing region which is the same and is as high as possible, and in order to achieve a constant relative speed between the printing head and the printing region.
This poses major challenges especially when printing non-rotationally symmetrical container shapes such as elliptical shapes. If such a body rotates about its axis, the distance between the surface to be processed and the axis of rotation changes constantly, as does the peripheral speed of the printing region as it passes the printing head. To achieve a good printing result despite the above issues, the rotational speed would have to be constantly adapted to the ever-changing speed conditions to ensure that the printing region always passes the printing head at the same peripheral speed.
The peripheral speed can be adapted to different container sizes or even to non-rotationally symmetrical container shapes by using controlled servomotors. The controller sets the rotational speed of the turntable to an effective constant relative speed between the surface and the printing head as said turntable passes the printing head.
However, the disadvantage of this solution is that it is comparatively expensive and complex to use servomotors and the appropriate control system. The necessary technology is also correspondingly complex.